1. Technical Field of the Invention
This invention relates to an auto-ignition internal combustion engine and, in particular to such an engine which is adapted to operate under lean burn conditions.
2. Description of the Prior Art
Normal internal combustion engines are generally of one of two types, spark ignition petrol or gas engines and diesel engines which use auto-ignition, that is, fuel is injected under pressure into an air charge that has been compressed to a very high temperature and pressure After mixing with the charge air and after some time delay, ignition occurs by way of auto-ignition.
Air standard cycles show that if an internal combustion engine could be operated with a gas charge of increased adiabatic index then it will be more efficient. The same air standard cycles show that an increase in the index of about 0.1 would result in an efficiency gain of approximately 10% at any compression ratio. One practical way to increase the adiabatic index is to operate the engine at leaner fuel-air ratios. This is because air has the highest index over any temperature range for unburned and burned charge and the index decreases as the fuel-air ratio increases. When a stoichiometric mix is reached the burned charge index starts to increase again, however, rich mixture combustion is less efficient overall due to the lack of oxygen for the complete oxidation of fuel.
If an engine could be operated with an equivalence ratio (the mass of fuel delivered in the charge relative to the stoichiometric mass of fuel) of approximately 0.5 an average increase in the adiabatic index for the burned charge would be in the order of 0.06 and approximately 0.02 for the unburned charge (Internal Combustion Engine Fundamentals, Heywood, 1988). At lower equivalence ratios the index would be higher. On this basis if an engine were built with increased capacity and combusting the same amount of fuel as its smaller counterpart an increase in output would occur.
Currently spark ignition engines operate with an equivalence ratio of 0.8 as a limit in the lean burn region. This is due to the increased occurrence of combustion failure at equivalence ratios lower than 0.8. There are three main cause for this: failure in the first instance of the spark to initiate a flame leading to misfire; the flame encounters regions in the charge too lean to sustain combustion; the flame speed, or combustion speed, is too slow to consume the charge before expansion quenches the flame.
Air standard cycles indicate that another means of increasing an engine""s efficiency is by increasing the compression ratio. The compression ratio defines the sum of the swept volume and combustion chamber volume divided by the combustion chamber volume. Air standard cycles show that an engine""s thermal efficiency would increase by an average of one percentage point for every unit increase in compression ratio from approximately 8:1.
There are three main practical limits to increasing compression ratio to any order. As the compression ratio increases the surface to volume ratio increases and beat losses become excessive. The heat losses start to cancel the efficiency gains at a compression ratio of about 17:1. As compression ratio increases the peak pressures increase so that structural considerations and necessary weight increases pose a practical limit. Commercially available diesel engines generally appear to have a compression ratio of not more than 30:1.
However, spark ignition engines are limited to compression ratios of approximately 8:1 to 10:1. This limitation is primarily due to the spark ignition model fuel""s tendency to xe2x80x98knockxe2x80x99. That is, the last portions of the charge not yet combusted (termed the end-gas) is forced to elevated temperatures and pressures whereby it auto-ignites ahead of the flame front. This auto-ignition is often of an explosive nature which sets up large spatial pressure differentials causing audible sounds to emanate from the cylinder. This knock phenomenon increases as the compression ratio increases Knock is to be avoided for two main reasons. Firstly, knock can cause considerable physical damage to the engine if the engine is allowed to operate in the knocking load region for sustained periods. Secondly, knock reduces efficiency mainly through noise and vibration.
The object of the present invention is to provide an auto-ignition internal combustion engine which overcomes or minimises the difficulties previously enumerated.
The invention, in the broad sense provides an internal combustion engine being an auto-ignition engine including means associated with the or each combustion volume whereby the compression ratio can be varied in response to the variation of other operating parameters of the engine to provide efficient functional operation under various conditions.